Making a flexible case for Metric 1-2-3 blocks

I’ve just ordered a matched pair of metric 1-2-3 blocks for use when machining components. For those of you have have not come across these before, they are a pair of hardened tool steel rectangles which have many uses as gauges, shims, bolt down blocks, etc.

A pair of metric 123 blocks

What's a 123 block?

The term “123 block” refers to the ratio of the dimensions of the blocks which were originally made in imperial measurements. Traditionally they were 1″ x 2″ x 3″ hence the term “123 block”. These are metric blocks but the same scaling principles apply as they are 25mm x 50mm x 75mm. Made from hardened tool steel and case hardened to around HRC 55-60. Each block is precision ground on all surfaces as a matched pair with squareness on all sides to within 0.00762mm per 25mm. Each block features 23 holes: 5 are tapped M10  the other 18 are untapped through holes which can be used to bolt the blocks together or to work surfaces.

Concept to Reality

The first step is to model the case in Fusion360, my preferred CAD package. Before I started on the case I took the opportunity to create a model of the 123 block itself. I suspect I will be making more jigs and brackets for these so it makes sense to create a model which I can use as a template in the future.

For the case, I need it to be a snug fit to the blocks and to fully enclose them but keep them physically separate. A simple two part clam shed design will work and I decide to use a flexible filament called TPU (thermoplastc polyurethane) for the 3d print. This is the same sort of material that flexible mobile phone cases are made from.

The source files are available for download from here;

 Metric 123 block storage case by Fatlab – Thingiverse

Case base showing fit of the 123 block
Raytraced and rendered model

"That looks real"

With Fusion360, not only can you model the components but you can also 3d render them in the materials you plan to use. In this image, the 123 block itself is set as High Carbon Tool Steel and the case material is set to Black Butyl Rubber which is similar in finish to the black TPU filament I plan to use.

When 3d printing with flexible filament, the elasticity of the material needs to be taken into account when dimensioning friction fit parts. For the block apertures I added 0.5mm to the overall dimensions for each block. The friction fit for the lid to the case is an exact fit (115mm x 85mm) for both the lid aperture and the base lip. The top and bottom thickness is set to 5mm with the overall case dimension being 125mm x 95mm x 35mm.

Printing the Case

I use a software package called Simplify3d to import the 3d model from the CAD software and generate the g.code which is sent to the 3d printer.

When preparing the 3d print, there are a number of parameters which need to be set but the important ones I will call out here are the number of layers for the walls, top and bottom faces and also the percentage infill for the solid areas. As I’m using flexible filament and I want to the case to provide a cushion for the blocks in the event of dropping it, I decided to print the unit with a wall thickness of 3 giving a solid face shown in blue of 1.2mm (3 x 0.4mm extrusion) for each inner and outer wall. The top and bottom faces shown in green are set to 4 layers which is 1.04mm total thickness (4 x 0.25mm layer height).

The thickness of the base and the lid are 5mm each and the sides are 10mm and 5mm respectively. This means that between the wall layers the printer creates infill shown in orange. In order to provide a cushion I used an infill percentage of 10% which allows for sufficient rigidity with flexibility.


Virtual test print in Simplify3d software
Both lid and base on the 3d printer

When 3d printing TPU it posed two main challenges. Firstly the flexible nature also manifests as elasticity which causes problems when trying to regulate extrusion. It’s a bit like a tube of glue which continues to ooze when you have stopped applying pressure. 

Secondly the print speed is much slower than the other filaments I normally use. In this case 30mm/sec compared to 60mm/sec for PLA. I printed the lid and the base separately and each took around 7 hours to print.

At the end of the day though, the benefits of the flexibility  is worth the extra time.

Showing how flexible the 3d TPU print is

Finished Product

The final finished case is a perfect fit both for the blocks themselves and the lid. I will cut and fit a piece of kitchen roll to the top and bottom of each aperture to soak up any excess oil and grease applied to protect the blocks from corrosion.

Overall a great success and I suspect I will be making more flexible cases for my other precision tools in the future

Both metric 123 blocks in place
Lid in place with a snug fit
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